Soundproofing a large closet with HVAC inside.

[Robert Adams]

Insulating the walls makes very little difference. I would insulate only if the walls were already open just for that tiny bit but I would not do it after the fact. 

         Upflow in the interior of the house are common here but most newer houses have them in the attic to reduce noise.... 
 
        Like mentioned a heavy door will help. Most of your noise comes from the return air grill because it is only a foot or so from the fan and nothing to stop the noise. Insulating the return plenum in sound absorbing and deadening material will do the most good but that is quite expensive to do. (I have looked into it) 

        I have added some baffles to block the direct line of sight of the sound and it does help a little. I threw some movers blankets in there test and they did attenuate the sound more than anything else I tried. 

       I have a solid door on ours and it is sealed. It's a 95% furnace so it has its own combustion air return. I have toyed with the idea of buying some of that acoustic foam and see if it does anything. Unfortunately though you really need a dense material to do the most good. Eiher a vinyl/foam material or the best is a combination of layers of foam vinyl and lead. Sheet lead is the best sound deadener.

[blackhat]

It will depend entirely on what is in the closet.  Might need combustion air, might need the fan tuned, might be nothing you can do without renovating duct work.  Call an HVAC man.

[Snipe Hunter]

When I was doing commercial work we would insulate ans double the drywall for sound proofing

We used to put rectifiers in closets for data center UPSs. We used fire rated doors, Rock Wool insulation and two layers of 5/8 fire rated sheetrock on both sides of the wall. It wasn't sound proof but it was close. Still, many furnaces have a second pipe for make-up air but the door may still need to be vented per code.

[frule]

Thanks to everyone for their replies. Sounds like I'll need to talk to the HVAC guy. This looks like it may be more complicated than anticipated!


Fred

[Cooler]

I stayed at a Holiday in in Newburgh New York about 40 years ago.  It was adjacent to the airport.  I watched as they were building the additional sections of the hotel (it was brand new).

They put 2" x 6" studs for the base and top of the walls and staggered the vertical 2 x 4s on 16" centers so that the sheetrock was attached on 28" centers on each side of the wall.  That isolated the two walls.  They then  wove felt carpet padding between the studs (that's why they staggered the studs) and stapled it in place.  The finally  put a second layer of sheet rock up over the first layer.   

I will say that I could not hear the jets as they flew overhead.  By  isolating the two sides of the walls and weaving the felt carpet padding they eliminated all the resonance.  

In your case I would make an effort to eliminate all the resonance in a similar manner.  I would hang sound absorbing foam (fire rated only) in the wall's interior.  
I would not block the louvers of the doors however as it may be needed for ventilation, oxygen source, etc.

There are sound insulating draperies and if you allow enough clearance from the door it might help too.

Make sure all interior materials are fire rated:  https://www.google.com/webhp?sourceid=ch...ing%20foam

[Robert Adams]

I forget what that wall framing method is called but it works very well. It also isolates vibration from going from outside to inside ad it also makes the wall more efficient because the studs aren't a direct thermal bridge from inside to outside.  


       The easiest foam to get would be melamine foam and the easy way to get it is from a boat supplier as it is used for sound insulation in boat engine bays. And the scraps can be given to the wife. Melamine foam is what those fancy white sponges are that are so pricey in the grocery store.

[daddo]

A lesson on soundproofing.

[Cooler]

I forget what that wall framing method is called but it works very well. It also isolates vibration from going from outside to inside ad it also makes the wall more efficient because the studs aren't a direct thermal bridge from inside to outside.  


       The easiest foam to get would be melamine foam and the easy way to get it is from a boat supplier as it is used for sound insulation in boat engine bays. And the scraps can be given to the wife. Melamine foam is what those fancy white sponges are that are so pricey in the grocery store.

Are they fire rated?  I've used "egg crate foam" that is the cheapest mattress topper.  It does a wonderful job of absorbing sound, but it is definitely not fire rated and I would not use it near a heat source.  (It is very cheap, though and will reduce noise of machinery if it is glued to some 1/4" ply and hung directly over the machine. Great for keeping shop noises from penetrating into the house.

The OP can put some furnace filters over the louvers of the doors.  The filters will allow air to penetrate and should (I imagine) reduce the amount of noise.

I had a 1967 Opel Manta and there was no fiberglass insulation on the hood like we used to see in American cars.  So I glued some on.  The engine immediately started running hot and I had to remove it.  So it is with trepidation that I am suggesting that furnace filters.  I don' t know if it will upset the cooling.

An alternative is active ventilation.  There are very quiet electronics cooling fans and the can direct air into the closet.  A dryer hose could direct it out.  And then a solid door could be installed.  I don't know how to specify the fan capacity though.

https://www.amazon.com/Electronic-Coolin...=306945011

[Robert Adams]

Melamine is a fire resistant foam. If you can use it in a boat engine compartment you can use it anywhere. The only thing more stringent than boats is airplanes. 

       It will melt before it burns. It's allot safer than many other materials that have fire retardant in it that will poison you  from its smoke. There is a reason that countries outside the US have banned fire retardants in foams and fabrics for household items. They create more problems than they solve. Canada bans most all of them and manufacturers have been caught selling furniture with fire proofing chemicals in it(ikea being one of the bigger ones caught) 


      Also for furnace clearance many of the newer ones that have efficiencies over 90% require 0" clearance on the back and sides and usually 3" on the front. Very little lost heat on them hence the pvc pipe for flue pipe.

[bottd]

Off of the OP track a little bit but the furnaces that use PVC pipe for the flue are because of the efficiently of furnace and the amount of heat they pull out of the flue gases by the addition of an additional heat exchanger. Now you have cool flue gases that will never put enough heat into the flue pipe to prevent condensation so you end up with condensate on the flue pipe and it is very corrosive, thus you use PVC.